Need help for LED driver LM3409

sagar saste1

Other Parts Discussed in Thread: LM3409

I need led driver for one of my illumination project so i decided to use LM3409. Now i want to drive 7 XP-G LED's with (Vf = 3.15, and I(led) = 1A) specification.
So i need following specification.
Vout = 21.05v
F(pwm) = 100 khz
Iled = 1A.

So which configuration i should use
1. EN PIN PWM DIMMING
2. EXTERNAL PARALLEL FET PWM DIMMING

over 9 years ago

0 Victor Chen_HK over 9 years ago

TI__Expert 5130 points

I think you can use either of them if you only want to achieve the dimming function. However, external parallel FET PWM dimming will have much faster response but needs more external components and has lower efficiency when the LED current is low as you extend the on-time of the shut FET.

Maybe you can start with the EN pin to see if you satisfy with the performance.

Regards,

0 sagar saste1 over 9 years ago in reply to Victor Chen_HK

Prodigy 120 points

how much frequency i can give using EN pin PWM dimming.

0 Victor Chen_HK over 9 years ago in reply to sagar saste1

TI__Expert 5130 points

You can use 20kHz as shown in the data sheet. I am not sure the exact minimum PWM dimming frequency, but I guess larger than 1kHz should be fine.

Best regards,

Victor

0 sagar saste1 over 9 years ago in reply to Victor Chen_HK

Prodigy 120 points

20Khz frequency can create noise which can be hear by human. So i am thinking to give maximum frequency. Near about 100KHz. Is it right?

0 Victor Chen_HK over 9 years ago in reply to sagar saste1

TI__Expert 5130 points

"In general, the maximum PWM dimming frequency using the EN pin should be at least one order of magnitude lower than the steady state switching frequency in order to prevent aliasing." The suggested frequency is even lower for good linear response.

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Need help for LED driver LM3409